The removal of particulate material from a liquid medium is a necessary step in many industrial operations. A typical example is the removal of solid particulates from various types of oils so that the oil may be reused. The oil to be recycled may, for example, be a lubricating oil, a hydraulic oil, a cutting oil, or an oil used in a steel rolling mill. Cutting oils used for working precious metals such as gold, silver, or platinum become contaminated with fine particles of the precious metal. It is economically imperative that the precious metal be recovered from the used cutting oil.
A filter apparatus for the treatment of used oil which is known in the prior art is disclosed in Japanese Patent Application Publication No. Sho 48-79357. The apparatus disclosed therein comprises a housing having disposed therein a filter medium comprising a plurality of hollow filter elements so arranged that a used oil to be filtered is sent under pressure from the top of the housing to the lower portion by passing through the hollow filter elements.
This apparatus has a number of deficiencies. In order to remove solid particulates adhering to the surfaces of the hollow filter elements of the apparatus, it is necessary to blow compressed air through the filter elements. This results in a low working capacity for the apparatus since the filtering operation has to be suspended while the particulates are being so removed. In addition, the efficiency of the filter medium is gradually lowered with operating time due to becoming plugged with accumulated solids. Moreover, the filter medium elements are likely to be damaged by the compressed air used to remove the solids.